Many kinds of pigments known in the art are used as colorants, for examples in paints, varnishes and inks. Generally such pigments have average particle sizes in the range of 0.1 to 10 microns but usually 1 micron or greater. To obtain these small sized particles, mechanical devices are often used to comminute solid particles into smaller ones. Ball mills, attritors, and bead mills and roll mills are commonly used for such purposes.
The use in a light valve suspension of particles produced by such mechanical methods is not practical, however, for several important reasons. First, the particles are usually too large, generally having an average size (diameter) of 1 micron or greater. Secondly, even if sub-micron sized particles are produced, the grinding and comminution process tends to make such particles spherical in shape or amorphous, reducing or virtually eliminating their aspect ratio. For use in a light valve suspension, as is well known in the art, anisometrically shaped particles are important; hence particles shaped like needles, rods or plates and the like are preferred and generally necessary because their anisometric shape facilitates orientation in an electric or magnetic field. Thirdly, an inherent disadvantage of mechanical comminution is that there is a large distribution of sizes, generally resulting in the presence of particles having a size of 1 micron or more even if the average particle size is less than 1 micron. Particles larger than 0.2 micron (one-half the wavelength of blue light) tend to scatter light, and such scattering increases exponentially with particle size. This fact and the fact that relatively large particles promote agglomeration are additional reasons why such comminuted particles are not desirable for use in a light valve suspension. There is thus a need in the art for light valves containing a light valve suspension of ultrafine particles.
Moreover, while various types of particles have been suggested in the prior art for use in light valves, heretofore it has not been practical to obtain particles of a submicron size and anisometric shape and good optical properties, except for polyiodide particles. However, in some cases polyiodide particles may not be sufficiently stable to ultraviolet radiation, and light valve suspensions of such polyiodide particles may degrade in terms of color and performance if exposed to intense ultraviolet radiation for a prolonged period of time unless UV absorbers are included in the light valve suspension. Also, nearly all polyiodide particles are limited to a blue color, whereas it is also desirable to have light valve suspensions which have off-state colors which are not blue. Accordingly, new types of particles for light valve suspensions are needed which will be suitably small and anisometrically shaped, which will tolerate high levels of ultraviolet radiation for long periods of time without significant degradation and/or which have a variety of off-state colors.